Plath wrote The Bell Jar chronicling her earlier suicide attempt

In an extract from the letter, Hughes tells Sagar that anti-depressants were to blame for her suicide.

Hughes wrote that "the key factor" that prompted Plath to gas herself at the age of 30 in 1963, was that she had mistakenly swallowed anti-depressants that gave her suicidal feelings.

Plath, an American poet whom he married in 1956, had taken anti-depressants before to similar adverse effect.

This long and important series of letters, written out in Hughes' dramatically vigorous hand, gives us an exceptionally direct insight

Chris Fletcher, British Library

Hughes wrote that a doctor had prescribed the unnamed drug for her without knowing what effect they would have.

"She was aware of its effects which lasted about three hours ... just enough time," an extract from Hughes' letter reads.

In another letter Hughes said that he had wanted a reconciliation with Plath following the estrangement just before her death.

But he complained that "stirrers and troublemakers complicated our getting back together in no small way", The Sunday Telegraph reported.

Other letters include descriptions of trips abroad and one written months before his death in 1998.

Chris Fletcher, curator of modern literary manuscripts at the British Library, where the collection will eventually go on public display, enthusiastically welcomed the donation.

"This long and important series of letters, written out in Hughes' dramatically vigorous hand, gives us an exceptionally direct insight into the complex mind of one of the most creative, charismatic and popular national literary figures of the post-war world," he said.

Birthday Letters

Plath and Hughes met at Cambridge University and married in 1956.

In 1998 Hughes published a collection of poems, called The Birthday Letters, which give his version of what has come to be regarded as one of the most tragic literary love stories of the century.

Many feminists and admirers of Plath hold Hughes responsible for his wife's suicide, accusing him of abandoning her for another woman at a time when she was emotionally unstable.

Hughes and Plath are two of the most read and studied post-war poets.